I think it’s an exaggeration to say we have “no idea” about dark energy — physicists like to say this to impress upon people how weird DE is, but it gives the wrong impression because we actually do know something about it. But not much!

Apparently the lack of spiral-galaxy wrap-up has been explained without dark matter recently. I know I found the article through the Discover blogs in the last few days, but I seem to be unable to track it down.

X

What we know about dark energy is that it is experimentally indistinguishable from a cosmological constant. The most puzzling thing about dark energy is how as the experimental data converges to agreement with the cosmological constant, the number of proposed novel theoretical models diverges.

Dutch Railroader

@X Only when you can explain the physical source of the cosmological constant and why it takes the value it does will the puzzle be solved.

Mandeep

That *is* a pretty cool way to accompany an audio track with illustrations indeed. Cham comes through again.

Only one semi-major mistake i saw — and i’ve seen a PhD winning particle physicist do the same in a talk, so there’s some edgimacation to be done it seems — the Bullet Cluster isn’t a collision of *galaxies*, but of two galaxy *clusters*. Quite different beasts.

(Technically: Somewhat the same effect must happen when galaxies collide as well, but it would be much harder to see because the gas in a galaxy is just not typically hot enough to glow in x-ray, because the gravitational potential isn’t high enough, because there isn’t enough mass in the galaxy to make it so).

http://amanda.uci.edu/~daniel/comic.html Daniel Whiteson

We originally planned just to do the comic, and Jorge came down to have lunch with us to discuss it; he recorded our lunch for his reference.

Later, we realized that the audio recording would work very nicely paired with the drawings. Since we didn’t plan to include the audio, the quality is a bit poor (you can hear us talking with food in our mouths!) and we spoke rather more loosely than we would have if we’d known it would be used…. but spontaneity beats accuracy.

spaceman

A few years ago I read an online power point presentation about the nature of dark matter. What amazed me then and still does is the sheer number of different dark matter theories and associated candidate particles: warm dark matter, cold dark matter, mixed-dark matter, axions, neutralinos, sterile neutrinos, gravitinos, etc. What many of the candidates seem to have in common is as follows: (i) motivation by physics outside of cosmology (e.g usually as potential solutions to a particle physics problem), (ii) proponents can cherry pick astrophysical observations that support “his or her” candidate, and (iii) the candidates’ respective parameter spaces are increasingly being narrowed by a variety of observations (or are they– look at the state of disagreement about what has been ruled out by the various direct detection experimental teams).

The take home message is that the field is still wide open and scientist A’s guess is as good as scientist B’s. Also, this state of confusion is before you throw in the modified gravity theories which are diverse bunch as well.

The situation is akin to one in which a crime has been committed and the case detectives each have their own theories as to who did it. Each detective can point to evidence that supports his “guy” just as each physicist can point to evidence that supports his particle. This aforementioned group of detectives at least agree that a human being is behind the crime. Now, there is another (minority) of case detectives who believe that a human being is not behind the crime just as there exists a minority of physicists who believe that a particulate matter is not behind the “missing mass problem.”

The only thing we know is that there is a “missing mass problem” which is like the case detectives saying that at least we know that a crime has been committed. Duh!!

Therefore, I would say that not only can we not claim to know much about DE, we can also not claim to know much about dark matter!

We know that dark matter exists and that it’s different from ordinary matter, which is a lot more than nothing. See, for example, Sean’s post about the Bullet Cluster from a few years back.

george briggs

dark matter has symmetry SU(1) unbroken, spin 0,repulsive gravity,

Anonymous

SU(1) is not much of a symmetry…

Gammaburst

The missing mass is Cavorite- hense the anti-gravity effect!

Shantanu

Sean, what do you think of Thooft’s latest paper?

http://washparkprophet.blogspot.com ohwilleke

Is there any very good reason to think that there is much about dark energy that we don’t know phenomonologically that is knowable? Very simple, homogeneous descriptions seem to be a very good fit to the observed dark energy phenomena.

I’m not sure that I share the “era of exploration”, “we’re just learning what questions to ask” attitude of the clip either. Most of what we don’t know seems to belong in the category of fine details and precise mechanisms behind phenomena we already understand pretty well.

It is a bit like we’ve cheated on some giant, super difficult math problem and know the question, and have the formula and have the bottom line answer, and even know what the “show your work” calculation on the last page of the problem that lead up to the answer look like, but don’t have the “show your work” calculations from the beginning of the problem, and might have some of the definitions that we used to understand the last few calculations a bit wrong.

No amount of scientific research at this point is going to change the bottom line implications of the strong nuclear force and weak nuclear force, which is that quarks are bound into nuclei that are only stable in a few isotypes of a subset of periodic table elements, and that unstable baryons and mesons decay in amounts and at rates that we already know empirically.

There is no good reason to think that unstable exotic particles and isotypes will have any engineering applications of consequence, and since the higher energy the particle is, the less stable it is likely to be, the more we push back the boundaries of what is out there the less likely it is to be useful.

We appear to have an essentially perfect understanding of electromagnetism (and by implication, all of the fundamental physics that are at the root of all of chemistry and biology), and the question marks in our understanding of gravity, dark matter and dark energy are unlikely to have any application other than making us feel better about how well we understand what we see in telescopes. General relativity is empirically validated well enough that any new discovery that helps us to understand gravity still have very little wiggle room to create observable effects of any great magnitude in the vast majority of places that we observe gravity at work.

The only stable particle we haven’t directly observed is a dark matter particle that by definition behaves more or less like a neutrino in terms of not having electro-magnetic or strong force interactions, while having gravitational interactions, and perhaps a Higgs.

I’d also note that the 5-1 dark matter ratio is predicated on an undercount of the amount of ordinary matter in elliptical galaxies that was corrected within the last year, and that after that correction the dark matter ratio is very close to a more interesting 1-1 ratio.

Maybe there is something one discovers from putting all the pieces together that allows us to do something interesting. But, at this point in the game of fundamental physics, the odds of new discoveries enhancing our understanding without having any practical application is very high.

george briggs

further on dark matter. It is the supersymmetric partner of the proton, and thus has a very long lifetime, so its decay has not been seen. since the proton is fermionic and spin 1/2, dark matter is bosonic and spin 0. This makes it scalar, in contrast to the proton, which is a simple point particle. Being bosonic, it has mutualy repulse gravity (dark matter entities repel each other, protons attract each other, and mixed sytems do not interact).

Joey

But why is everybody on the video white (and mostly male)?

spaceman

Kevin,

Although I do not have the educational background to make an informed decision as to whether or not the Bullet Cluster is proof that dark matter exists, I do know that there are those in possession of such a background who claim that the Bullet Cluster data may be consistent with theories that do not include additional matter. Also, the only way we can say that we proved the existence of dark matter is if we detect the stuff directly because indirect evidence can always be subjected to the objection that there may be other explanations for the phenomena with evoking extra matter. All of the evidence is indirect and inconclusive and there are those in the community who have alternative theories for the same observations.

TomC

Just a reminder that—as has been discussed in these pages many times—even if the rotation curves of galaxies were to be explained by something other than dark matter, you would still have to explain the ratio of peaks in the angular power spectrum of the microwave background. That is an extremely robust measurement of the ratio of normal matter to total matter in the universe, and it agrees perfectly with the ratio that explains the other effects discussed here.

george briggs

I finish on dark matter, It alwayes appears in naturre together witn an equal amont of ordnary, fermionic matter. The combination is a strange new form of matter as yet unnamed.I like to cll it fermibosonic matter. It has also been identified by A.G.Lisi (see Scientific American, Dec.2012) and found to have the exceptional symmetry type E8. This new type of matter has no net gravitational energy and appears to be a massless entity for all extent and purpose. Enter Roger Penrose and his new book “Cycles of Time” which I have not been able to get a copy of yet. If I get it right, the fermibosonic matter entered our Universe from a immediately prior universe (after being cleaned up to remove its metalicty and reduce its entropy).

Matt B.

George, protons do not attract each other (like charges repel). The idea of dark matter having negative mass is one I like, but I haven’t seen it explored.

george briggs

mMatt, we need tg get rolling with negativee mass. The idea has been around since peter higgs. Equal amounts of both types in one fERMIBOSONIC ENTITY add up to zero, i.e. the entity is massless. There is no need for a Higgs Mechanism to get mass,- just supply a particle of ordinary matter combined with dark matter, which I call a fermibosonic entity and which has been identified as of E8 symmetry by A.Garrett Lisi

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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson.
Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .